The invention relates to a process for the optical testing of a surface of an object, in which the surface is illuminated by at least one light source and the light reflected and/or scattered by the surface is projected onto at least one substantially flat, light-sensitive element of a light-sensitive receiver. The receiver consists of several pixels in the form of a grid in the line direction and the space direction. In a test phase at least one actual image is generated that is compared with at least one desired image which was produced in a read-in phase. Nevertheless, the suitability of the invention for other surfaces will be recognised.
CDs have recently become increasingly popular as sound media for home use because of the high potential sound quality, and as pure data media in data processing because of the high potential data density. They are therefore a mass-market product, but have to meet high to very high quality demands, particularly when used as pure data media.
Generally speaking a CD comprises a circular disc, multi-layered in cross-section, with a central tap hole for fitting and centring in a player. From the bottom, i.e. viewed from the read side of the player, upwards the CD consists of a transparent plastic layer which contains all the data in the form of pits, a thin metal layer, generally of aluminium, for metallizing the plastic layer, and a thin lacquer layer which is usually hardened by UV light, for protecting the metal layer. The imprint, the so-called label, to provide information to the consumer, is then applied to the UV lacquer layer by means of known printing processes.
In the radial direction, in a CD several coaxial, circular regions which move outwards from the tap hole can be distinguished. Directly at the tap hole is the region used to fit the CD in the player. Adjoining this is a region in which the so-called ident code, by means of which the CD can be unequivocally identified, is impressed. There then follows the region used for the actual data storage. If the CD is recorded up to its maximum storage capacity the region ends directly on the outer edge region. Otherwise the so-called lead out or a reflective strip is arranged between the edge region of the CD and the data region.
In the manufacturing process a polycarbonate blank is initially produced by the die-casting method, wherein all data are already impressed by the die. One surface of the blank is then provided with the metallic reflective layer by the sputter process and sealed with the lacquer layer. In this process the CD is centrifuged in order to achieve a uniform distribution of the lacquer layer and as thin a layer as possible. The label is then printed on the CD.
In principle the label is of no importance to the function of the CD because a CD is read from the underside. For the consumer, however, a perfect label is often a criterion for perfect playback of the CD so that it is necessary to produce a faultless imprint on the surface of the CD. In contrast the UV lacquer layer on a CD must always be perfect because otherwise there is the risk of the underlying metal layer oxidizing prematurely, which could cause reading errors. Processes with which the surface of the CD, i.e. the label and optionally also the UV lacquer layer, can be tested are therefore required.
Generally speaking the surface of the CD is printed in any manner. Colour surfaces, pictures, inscriptions or the like can be applied with the most varied colour application processes for example. It is of course also possible for the CD to be partially unprinted, so that the metal layer is visible through the UV lacquer from the top. Furthermore a CD is often neither printed nor provided with a metal layer on the inner and outer edge region and is therefore transparent in those regions.
Generally speaking an optical process is used for testing the printed surface of the CD, in which the surface of the CD is photographed by a light-sensitive receiver in the top view from above. In a data processing unit the actual image taken is compared with a previously read-in desired image of a perfect surface with certain test criteria. Any deviation is then detected as a fault and the CD is graded according to the size and nature of the deviation.
With automatic test methods, electronic CCD cameras which convert the light received directly into electrical signals are usually used. The substantially flat light-sensitive element of a CCD camera. is constructed in the form of a grid and has a multiplicity of pixels in line and space direction. At the same time this grid is a measure of the camera's resolution. In conventional CCD cameras the light-sensitive element has approx. 750 pixels in the line direction and approx. 580 in the space direction, for example, and the resolution is approx. 0.22 mm.
It has, however, been shown that this resolution is often inadequate for testing a surface with the desired criteria. In particular, different regions require different criteria, as plane imprints must meet different criteria, for example, from regions with sharp light/dark contrasts, such as are produced by lettering. In the case of plane imprints, for example, with metallic prints there is the problem that the metallic effect is achieved by means of metal particles whose content in the imprint is irregular. With such imprints in particular it is possible for an imprint that is perfect to the human eye to be detected as a fault by the test method. With low-contrast imprints, such as solely background images, there is the risk that the boundary lines will not be detected perfectly so that potential faults cannot be identified.
Particularly where electronic CCD cameras are used there is a further problem in that a pixel actually has only a limited photo-sensitive region which is surrounded by a region that is not photo-sensitive. Tracks are then arranged in this region for example. This means that the surface is not photographed in its entirety and the immediate ambient field of an exposed pixel not detected at all.
In principle it is of course possible to improve the test by increasing the resolution of the camera used. Increasing the resolution is not, however, possible without expense. Furthermore the fact that the light-sensitive element would have substantially more pixels, thereby prolonging the computing time required to test the surface, is a disadvantage. In addition the light-sensitive receiver will then be larger in size and can no longer be readily incorporated in the test device which often needs to have a limited structural volume because it is incorporated into existing production plants.